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Low Temperature Synthesis of Hexagonal Shaped α-Al2O3 Using a Solvothermal Method
This study demonstrates the low temperature synthesis of α-Al2O3 by solvothermal method using gibbsite alumina precursor in 1, 4-butanediol solvent according to various pH conditions. In acidic solution, an orthorhombic boehmite (AlOOH) structure was obtained after solvothermal reaction. A significant result in this study was that the solvothermally synthesized alumina in at 300 °C for 36 h represented a rhombohedral α-Al2O3 structure hexagonal shaped with about 1.5~2.0 μm of particle size. Otherwise, the α-Al2O3 structure was rather changed to the mixture of a boehmite and α-Al2O3 structures above . In the case of α-Al2O3 synthesized at , the specific surface area was 26.18 m2/g, and the particles that were stable in acidic solution resulted in 61.80 mV of zeta potential.
The α-Al2O3 powder has considerable potential for a wide range of applications including high strength materials, sapphire crystal growth, electronics, semiconductors, and catalysts [1–4]. In particular, the current high demand for sapphire substrates for the LED market has greatly added to the crystal growth business on top of existing applications. The volume of sapphire wafer production is currently measured in hundreds of millions of units annually. Here α-Al2O3 crystals have attracted attention as a source of sapphire. Due to its versatility, increasing interest has focused on the synthesis of α-Al2O3. Conventional synthesis processes of α-Al2O3 involve vapor phase reaction, precipitation, sol-gel, hydrothermal, and combustion methods. Vapor phase reaction for preparation fine α-Al2O3 powder from a gas phase precursor demands high temperature above 1200°C . The precipitation method suffers from its complexity and time consuming as like long washing times and aging time . The direct formation of α-Al2O3 via the hydrothermal method needs high temperature and pressure . The combustion method has been used to yield α-Al2O3 powders, whereas the powder obtained from the process is usually hard aggregated but contains nano-sized primary particles . Sol-gel a commonly from a precursor solution used technique, involves the formation of an amorphous gel; however, this method needs some thermal treatment steps at various temperatures [9, 10]. In general, α-Al2O3 derived from the decomposition of gelatinous boehmite, gibbsite, and related hydroxide alumina undergoes a number of transitional phases. Amorphous alumina dehydrates at 500°C to form γ-alumina which then transforms to δ-alumina and θ-alumina before becoming α-Al2O3 in the range of 1,200~1,400°C, depending on the procedure . Recently several studies on the preparation of α-Al2O3 have tried to lower the formation temperature by using additives [12, 13]. It has been suggested that the metal-organic-derived alumina could lower the transformation temperature of α-Al2O3. In our previous researches [14, 15], we have also reported two papers; both were about mild temperature synthesis of α-Al2O3 using a chelating reagent as like ethylenediamine and a premicrowave treatment, resulted that α-Al2O3 structure was successfully obtained below 1,000°C. However, the crystallization temperature is still high. Therefore, new methods are needed to overcome the problem. In particular, the solvothermal method is an alternative to calcinations for promoting crystallization under mild temperatures. This process has been applied recently to the synthesis of small ceramic particles and films containing a range of metal oxides. Solvothermal treatments can be used to control the grain size, particle morphology, crystalline phases, and surface chemistry by regulating the sol composition, reaction temperatures, pressure, solvent nature, additives, and aging time. The particles prepared using the solvothermal method are expected to have a larger surface area, smaller particle size, and greater stability than those obtained by other methods, such as the sol-gel method. There are few reports of α-Al2O3, which are prepared using the solvothermal method. Bell and Adair  have synthesized α-alumina in 1,4-butanediol solvent by reaction of gibbsite powder at 300°C for 36 h under a stirring rate of 460 rpm and autogenously pressures. They have also controlled the morphology through the use of special adsorbents in non-aqueous solution synthesis involves consideration of solvent degradation. Such studies are continuing, but without evident success as yet.
In this study, we have also tried to synthesize a special α-Al2O3 powder as a raw material for sapphire crystal growth in application to the LED industry. The α-Al2O3 structure in this study is controlled according to the effects of the pHs in the preparation step using a solvothermal method. The as-synthesized Al2O3 powders are characterized by X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), specific surface areas (Brunauer-Emmett-Teller, BET), X-ray photoelectron spectroscopy (XPS), and zeta potentials using electrophoresis light scattering (ELS) measurements.
A solvothermal method was introduced in this study . Gibbsite (Al(OH)3, sigmaaldrich.com/sigma-aldrich/home.html) was used as an aluminum precursor. First, the Gibbsite of 10.0 g was well dispersed in small amount of methanol with sonification for 1 h and then the colloidal solution was well mixed with excess 1,4-butanediol solvent for 2 h. The colloidal solution was thermally treated at 70°C for 24 h to remove methanol. Then acetic acid or ammonia water was slowly dropped into the solution to control pH = 4, 7, 9, and 11. The final solution was stirred homogeneously for 5 h, and then the solution was moved to a liner in autoclave, and finally the autoclave was heated in N2 atmosphere at a rate of 10°C/min to 300°C and then maintained isothermally at this temperature for 36 h and 70 atm. After solvothermal treatment, the obtained white powders were washed and dried. The synthesized Al2O3 were named to pH 4, 7, 9, and 11 according to pH of the final solution in the preparation step.
The as-synthesized Al2O3 powders were identified using powder XRD (model MPD, PANalytical, Yeungnam University Instrumental Analysis Center, Korea) with nickel-filtered CuKα radiation (30 kV, 30 mA) at 2-theta angles of 10–90°. The scan speed was 10°/min, and the time constant was 1 s. The sizes and shapes of the materials were measured by field emission SEM/energy-dispersive X-ray spectroscopy (FESEM/EDS; S-4100, Hitachi, Yeungnam University Instrumental Analysis Center, Korea). XPS measurements of the binding energy of and orbitals in alumina powders were recorded with a model AXIS-NOVA (Kratos Inc., Korea Basic Science Institute Jeonju Center, Korea) system, equipped with a nonmonochromatic AlKα (1486.6 eV) X-ray source. The specific surface area was calculated according to the BET theory that gives the isotherm equation for multilayer adsorption by generalization of Langmuir’s treatment of the unimolecular layer. The BET surface areas were measured using a Belsorp II instrument. The materials were degassed under vacuum at 120°C for 1 h before the BET surface measurements. Then the samples were thermally treated at 300°C for 30 min. The BET surface areas of the materials were measured through nitrogen gas adsorption using a continuous flow method with a mixture of nitrogen and helium as the carrier gas. The zeta potentials of the materials were determined by electrophoretic mobility using an electrophoresis measurement apparatus (ELS 8000, Otsuka Electronics, Japan) with a plate sample cell. ELS determinations were performed in the reference beam mode at a laser light source wavelength of 670 nm, modular frequency of 250 Hz, and scattering angle of 15°. The standard error of the zeta potentials, converted from the experimentally determined electrophoretic mobility, was typically < 1.5% and the percent error < 5%. To measure the zeta potentials, 0.1 wt% of each sample was dispersed in deionized water and the pH of the solution was adjusted to 7. Relative sintered particle size distributions of the various pH solutions were also measured by using this equipment. Thermal gravimetric analysis measurements were collected using a TGA N-1000 (Scinco. Korea) thermal gravimetric analysis (TGA) equipped with a platinum crucible. Samples were heated from room temperature (~50°C) to 900°C with a heating rate of 5°C min−1 while the chamber was continuously purged with O2 gases at a rate of 25 mL/min.
3. Results and Discussion
Figure 1 shows the XRD patterns of the four types as-synthesized Al2O3 powders according to various pH conditions after solvothermal treatment at 300°C for 36 h. The synthesized Al2O3 powder at pH = 4 exhibited peaks at 2 theta angles of 14.51, 28.22, 38.44, 49.18, 49.47, 55.19, 63.93, and 72.03° corresponding to the (d020), (d120), (d031), (d200), (d200), (d220), (d231), and (d251) spaces, respectively, and they were ascribed to the orthorhombic boehmite (AlOOH) structure . However, the structures changed to the boehmite/α-Al2O3 mixture at pH = 7. A significant result showed at pH = 9 that the XRD peaks assigned to α-Al2O3 were clearer and sharper without high thermal treatment above 1,000°C. The synthesized α-Al2O3 particle exhibited peaks at 2 theta angles of 25.57, 35.14, 37.76, 43.33, 46.16, 52.53, 57.47, 61.27, 66.49, 68.18, and 76.84, corresponding to the (d012), (d104), (d110), (d113), (d202), (d024), (d116), (d018), (d214), (d300) and (d1,0,10) spaces, respectively . It was ascribed to the rhombohedral structure. Consequently, α-Al2O3 was easily acquired with the solvothermal process, and it lowered the existing temperature by 500~900°C, compared to the sol-gel process. These results revealed the pH effect on the crystallinity because of hydrolysis and poly-condensation in solvothermal treatment. This indicates that the rate of hydrolysis is governed by the hydronium ion in acidic solutions, so that the amount of water is small due to rapid formation of . On the other hand, the reaction is controlled by the hydroxyl ions when powders are derived at pH = 9. The initial growth leads to a linear chain, but the high concentration of ions leads to crystallization because the probability of intermolecular reaction is higher than the intra-molecular reaction. The most probable metal-oxygen polymeric network, formed at high pH. The full width at half maximum (FWHM) height of the peak at 2θ = 35.14° (104) was measured, and the Scherrer equation , , where the wavelength of the incident X-rays, , the FWHM in radians and the diffraction angle, was used to determine the crystalline domain size. The calculated crystalline domain size based on a special peak of 35.14° (d104) is 39.66 nm for the α-Al2O3 powders synthesized at pH = 9.
TEM and SEM images of the solvothermally synthesized Al2O3 particles prepared at various pHs are shown in Figure 2. This figure revealed that the shapes varied according to the pH. At lower pH, the Al2O3 powder was close to an orthorhombic rectangular shape. However, at neutral solution, the form was changed to cube/hexagonal mixture shape. In particular, it was transferred the α-Al2O3 structure to the perfect hexagonal-shaped at pH = 9, and eventually it became huge square pillars of 1.5–2.0 μm. However at higher pH, the shape was seemed to collapse.
The atomic compositions on the surface of the synthesized α-Al2O3 powders were analyzed by EDS and the results are summarized in Table 1, which revealed the presence of Al and O as the only elementary components of two samples with an Al : O atomic ratio of about 6 : 4. In the samples synthesized in acidic solutions, the measured Al : O ratio revealed a little higher aluminum content than that in alkali solution of pH 11 in this study. However, at pH = 9 that represented α-Al2O3 structure, the oxygen concentration was higher compared to the other phase’s alumina.
Figure 3 presents the high-resolution spectra obtained from the quantitative XPS analyses of the as-synthesized four Al2O3 samples produced at various pHs. The survey spectra of materials contained and peaks by XPS handbook . It is well known that the orbital in α-type Al2O3 has a binding energyof 73.5~74.2 eV, and this was assigned to Al3+ in Al2O3 and was almost same in pH 4, 7, 9, and 11, which gave an orbital binding energy of 72.85~73.29 eV. In general, a large binding energy indicates the presence of more oxidized states. In this study, the orbital binding energies were shifted to lower binding energy compared to the pure α-type Al2O3 represented in XPS handbook, which were assigned to reduced Al ions in Al2O3. The O1s region was decomposed into two contributions: metal–O (~530.2 eV) in the metal oxide and metal–OH (531.0 eV). In general, a higher metal–OH peak indicates that particles are more hydrophilic. However, only a single peak was seen in all samples induced from pH 4, 7, 9, and 11, at around 531~532 eV, which was assigned to Al–O.
The BET surface areas of the materials were measured by nitrogen gas adsorption using a continuous flow method with a mixture of nitrogen and helium as the carrier gas. The pore size distribution is an important characteristic for porous materials. The relative pressure at which pore filling takes place by capillary condensation can be calculated from Kelvin’s equation. By using Kelvin’s equation, the pore radius in which the capillary condensation occurs actively can be determined as a function of the relative pressure . The mean pore diameter, , was calculated from , where VT is the total volume of pores, and the BET surface area. All isotherms belonged to type I–V according to the IUPAC classification . The adsorption-desorption isotherms of N2 at 77 K for the four Al2O3 powders, were calculated as shown in Figure 4, and the values are also summarized in the table. All of Al2O3 samples in this study showed isotherms belong to III type in the IUPAC classification . The synthesized Al2O3 samples in this study did not have any pores and however the BET surface area and pore volume were decreased in Al2O3 sample synthesized at pH = 9 to 26.18 m2 g−1 and 0.249 cc/g, respectively, compared to the other Al2O3 samples. We expected from this result that the α-type Al2O3 derived from pH = 9 has a larger particle size and higher density.
The zeta potential is an important parameter in colloidal stability, since it reflects the variation in surface potential for a specific material. These powders were derived from the solution route at low temperature; therefore, zeta potential studies were conducted to understand the surface charge of these powders. Figure 5 shows the zeta potential data of an aqueous suspension of synthesized Al2O3 powders. No electrolyte was added to control the ionic strength of the solutions. The zeta potentials of all of the Al2O3 suspensions were significantly decreased with increasing pH. The surface charges were transferred from positive in acidic solution to negative in alkali solution. For α-Al2O3 synthesized at pH = 9, the isoelectric point was at pH = 8 with large aggregation, and it positively charged to a maximum of 61.80 mV at pH = 3, which indicated that the α-Al2O3 colloidal existed stably while having a small aggregation. Above this pH, the positive charge of the α-Al2O3 particle was decreased with the same trend of mobility, resulting in an average aggregated diameter of 2,555 nm at pH = 7.
This study demonstrated the effect of pH in solvothermal synthesis to reduce the crystal growth temperature of α-Al2O3 compared to ordinary methods. Most significantly, the solvothermal treatment produced rhombohedrally structured α-Al2O3 with the hexagonal particle range of 1.5~2.0 μm at 300°C with pH = 9 and however the boehmite and mixed structures were seen at pH = 4, 7, and 11. The surface area was smaller in α-Al2O3 synthesized at pH = 9. Electrophoretic light scattering (ELS) measurement in aqueous solution at pH = 3 revealed positive surface charges in the α-Al2O3, which indicated that the α-Al2O3 colloidal existed stably while having a small aggregation.
This research was financially supported by the Ministry of Education, Science and Technology (MEST) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation.
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